1. Field of Invention
The present invention relates generally to circuits and methods that involve multiple voltage domains. More specifically, the present invention relates to circuits and methods for transferring signals across a voltage domain boundary.
2. Discussion of Related Art
Processor circuitry often includes multiple logic domains, wherein the logic domains may be distinguished by a difference in voltage and/or clocking speeds. For example, a processor may comprise a domain that utilizes “high voltage” logic, and another domain that utilizes “low voltage” logic, the terms high voltage and low voltage being used herein merely to distinguish the two. The voltage difference of the domains may take any value. If the processor operation requires communication between logic components belonging to different voltage domains, circuitry must be provided to enable the communication. The circuitry may take the form of level shifters, which function to shift the level of a signal from a low voltage to a high voltage, while maintaining the state of the signal, if the signal is intended to cross the boundary from a low voltage domain to a high voltage domain. Similarly, level shifters may operate to shift the level of a signal from a high voltage to a low voltage, while maintaining the state of the signal, if the signal is intended to cross the boundary from a high voltage domain to a low voltage domain. It is noted that the terms “domain” and “domain boundary” as used herein do not necessarily imply a physical or geometrical structure or division. Rather, the term “domain” is used to distinguish between types of components.
An example of a known processor implementing logic domains of differing voltages is the ADSP-BF535 processor, produced by Analog Devices, Inc. Communication is required between logic components in the different domains. In this processor, one level shifter is provided for each bit that is to be transferred from the low voltage domain to the high voltage domain, and from the high voltage domain to the low voltage domain. In essence, the configuration allows parallel transfer of data bits across the voltage domain boundary. In order to transfer a large number of bits in parallel by this method, many level shifters must be provided. Since level shifters can be expensive in terms of occupied chip area and power consumption, it is desirable to minimize the number of level shifters used. Thus, a need exists for space-saving and energy efficient circuits and methods that enable the transfer of signals across a voltage domain boundary.